Tuesday, December 14, 2004

Decryption by Clancy

The phone they spoke over was the Russian version of the American
STU-3, the technology having been stolen about three years before by a
team working for Directorate T of the First Chief Directorate. The
internal microchips, which had been slavishly copies, scrambled the
incoming and outgoing signals with a 128-bit encryption system whose
key changed every hour, and changed further with the individual users
whose personal codes were part of the insertable plastic keys they
used. The STU system had defined the Russians' best efforts to crack
it, even with exact knowledge of the internal workings of the system
hardware, and they assumed the the Americans had the same
problems—after all, for centuries Russia had produced the
world's best mathematicians, and the best of them hadn't even come up
with a theoretical model for cracking the scrambling system.
But the Americans had, with the revolutionary application of quantum
theory to communications security, a decryption system so complex that
only a handful of the "Directorate Z" people at the National
Security Agency actually understood it. But they didn't have to. They
had the world's most powerful supercomputers to do the real
work. They were located in the basement of the sprawling NSA
headquarters building, a dungeonlike area whose roof was held up with
naked steel I-beams because it had been excavated for just this
purpose. The star machine there was one made by the company gone
bankrupt, the Super-Connector from Thinking Machines, Inc., of
Cambridge, Massachusetts. The machine, custom build for NSA, had sat
largely unused for six years, because nobody had come up with a way to
program it efficiently, but the advent of quantum theory had changed
that, too, and the monster machine was now cranking merrily away while
its operators wondered who they could find to make the next generation
of this complex machine.

I doubt anyone could turn a souped-up ConnectionMachine into a quantum computer. But one could imagine some smart
NSA scientists finding a way to simulate a variation of Shor's quantum factoring
algorithm on such a device, well enough to break moderately long RSA
codes. Just thinking.

3 comments:

Feynman showed that quantum mechanics can be simulated efficiently by quantum computers but not by classical computers. Specifically, quantum computers suffer at most a polynomial slowdown, while classical computers suffer an exponential slowdown. This implies that classical computers cannot efficiently simulate quantum computers, which helps explain the buzz about recent developments in quantum computation.

Thus, implementing Shor's quantum algorithms on a classical computer will be inefficient, and most likely worse than a more direct attack. As it happens, it's been done, and while the author of that paper doesn't seem to have included performance numbers, it's clear from considering the data structures that they are fairly poor.

Second, a six year old system along the lines of the Connection Machine line wouldn't be state of the art today. Off the shelf processor speeds, node interconnect speeds, memory and hard drive sizes, etc., have improved sufficiently in the intervening time that the idea that the NSA would be using such a system is implausible. Far more likely that they would buy a bigger and better Blue Gene or the like.